I was with my father once when he ran into an old RIETS classmate who had never gone into the Rabbinate. "By the way, did YU ever revoke your semikha?" he asked. My father didn't know what he was talking about. Turns out after a major scandal involving someone with Orthodox semikha (not a rabbi, and not a YU product), YU had sent notice to all non-practicing ordainees that they couldn't call themselves "rabbi" anymore. (My father was a rebbe for many years, which may be why he didn't get the notice and/or slipped under radar.)

It's technically not a revocation of the actual semikha, and it didn't have real power- anyone can call themselves anything. I think it basically meant that if you got into trouble, YU could deny that you had semikha from them, and in practical matters it probably meant that you couldn't do state-related clergy things like marry people or claim tax deductions (if anyone checked).

Of course, that doesn't cover *practicing* rabbis. There the consequence would be getting thrown out of the RCA or something.